clarified comment to CStruct#to_s

ext/generate_structs.rb

@@ -50,7 +50,7 @@
     fd_def.puts "PKCS11_DEFINE_STRUCT(#{struct_name});"
     fd_doc.puts"class PKCS11::#{struct_name} < PKCS11::CStruct"
     fd_doc.puts"# Size of corresponding C struct in bytes\nSIZEOF_STRUCT=Integer"
-    fd_doc.puts"# @return [String] Binary content of the C struct\ndef to_s; end"
+    fd_doc.puts"# @return [String] Binary copy of the C struct\ndef to_s; end"
     fd_doc.puts"# @return [Array<String>] Attributes of this struct\ndef members; end"
 
 		# try to find attributes belonging together